A scanner has been more and more popular so as to gradually become a standard peripheral equipment of a personal computer. Therefore, scanner manufactures have been trying their best to develop a more and more powerful and/or competitive scanner. For example, a minor problem for the scanner derived from the illuminance stability of a light source of the scanner is also worthy to be seriously considered and improved.
The light source commonly used in an image scanner is a white fluorescent lamp such as a cold cathode fluorescent tube (CCFT). As known to those skilled in the art, the illuminance of a CCFT generally increases gradually when lighting up, and becomes stable after a couple of minutes. Therefore, the lamp has to be warmed up before use in order to obtain stable illuminance for stable scanning quality. Please refer to FIG. 1 which shows an illuminance variation of a CCFT having a model no. of FL-41266 (AE) and manufactured by Toa Elevam Co. (Taiwan). As shown in FIG. 1, a constant tube current (I.sub.L) of 6 mA is expected to be provided for the CCFT to perform the scanning operation of the scanner. The brightness of the CCFT, however, varies with time at the outset of lighting up. During this unstable stage, as called a warm-up mode, the scanner is not allowed to enter a scan mode to normally work because of possible errors resulting from the variable illuminance. Furthermore, for some conventional scanners, the CCFT is put out after a predetermined standby period whenever a scanning run is complete. Therefore, the CCFT has to be re-started to illuminate, and another warm-up period has to be waited for if another scan run is to be performed. In brief, whenever the CCFT is switched from an off-state to an on-state, a warm-up mode is re-started, and another couple of minutes has to be waited for. Consequently, the overall scanning speed of the scanner is unsatisfactory.